In typical low temperature fuel cells, of considerable interest in the automotive field, hydrogen (H2), or an organic material with high hydrogen content, supplied to the anode, is oxidized with the release of electrons, whilst at the cathode, oxygen (O2) is reduced. Platinum (Pt) is a highly active material acting as a catalyst in such fuel cells, and can be used at both the cathode and anode. At the cathode, platinum catalysts are commonly employed to assist in the oxygen reduction reaction (ORR).
Conventional fuel cell catalysts are typically based on Pt or Pt-alloy nanoparticles deposited on carbon supports. However, the high cost of these materials and limited electrochemical stability impede their use in commercial fuel cell powered devices. Very fine platinum (Pt) particle dispersions (1 to 2 nm) have been considered to minimize the precious metal loading without losing catalyst activity. However the ORR activity of Pt-catalysts is particle size dependent: the maximum mass activity is obtained with 3 to 4 nm Pt particles. An optimal mass/cost benefit with an ultra-fine Pt dispersion has thus not been achieved. A Pt (or Pt-alloy) particle deposited on a carbon-type support is illustrated in FIG. 1. Such a system has a maximum mass activity and specific activity for particles of 3-4 nm, impeding the cost-effective reduction of Pt loading via a finer dispersion (particle diameter <3 nm). The Pt-based catalyst suffers from dissolution and sintering, while the carbon support is subject to corrosion.
Recently, to reduce the Pt loading by enhancing Pt utilization and activity, some groups have developed core-shell nano-catalysts based on a precious metal core or a base metal core (such as nickel). With a precious metal core, the resulting material is still expensive, but a base metal core risks being unstable in an electrochemical and acid environment.
Examples of core-shell type catalysts are described in US20100197490, US20070031722, and US20090117257. This type of system is shown schematically in FIG. 2, being based on a Pt shell and a noble metal core. This enhances the catalytic activity but does not provide reasonable cost. A core-shell type catalyst, as described in CN101455970 and shown schematically in FIG. 3, based on a Pt shell and a transition metal core, enhances catalytic activity but does not provide reasonable electrochemical stability. The core-shell type catalyst described in US20060263675, and shown schematically in FIG. 4, based on a Pt shell and a bronze WO3 core, offers a possible solution to the durability issue, but no evidence is provided. With a Pt shell limited to very few atomic layers (<3) the ORR activity could suffer a large overpotential, due to a strong interaction between the oxide substrate and the Pt overlayer.